The operating principle of the refractometer has been known for more than one hundred years. These days, refractometers are used in many fields. For example, refractometers are used in the food industry, wood-processing industry, chemical industry, and various types of research activities.
A refractometer measures the refractive index of a solution in an optical window by means of the total reflection created at the interface between the measuring surface in the measuring window and the solution being measured. A beam of rays from a light source is directed to the interface between the measuring window and the solution being measured. A part of the beam of rays is entirely reflected off the solution, a part is partially absorbed into the solution. This creates an image in which the location of the boundary between the light and dark areas depends on the critical angle of the total reflection, so therefore on the refractive index of the solution.
Known refractometer implementations are described in closer detail in several publications, such as U.S. Pat. Nos. 6,067,151, 6,760,098, and 7,619,723, for example.
In known implementations, the measuring window of a refractometer has been prism-like. The entrance surface, measuring surface, and exit surface of the beam of light have been plane surfaces. To focus light, separate optical components, lenses, have been used. Due to the light passing through a plurality of optical surfaces, reflections are created, and because optical surfaces are not optimal, blurring is also created.
When light arrives from an optically denser substance to an optically less dense substance through a plane surface, the arriving beam of rays is expanded so that the rays arriving against the plane surface at the largest angle are refracted the most. This causes non-linearity in the angular distribution and refractive index measurement of light.
It is known that the numerical aperture remains the same when light travels from one material to another through successive surfaces unless the interfaces have optical refractive power. In a measuring window in which light travels through plane surfaces which thus do not have optical refractive power, the numerical aperture is the same for the rays entering the prism and those exiting it. The specified numerical aperture is defined from the measured refractive index area and the refractive index of the material of the measuring window from the formulaNA=ni sin θ,where ni is the refractive index of the material of the measuring window and θ is the angle between the middle ray and edge ray or the measurement area.
A refractometer measurement involves the analysis of the image created by the reflection of light. The purpose of the image analysis is to find the critical angle of the total reflection, which is the boundary where a light area of an image formed as described in the above turns into a dark area.
From at least U.S. Pat. Nos. 4,571,075 and 5,309,288 it is known to manufacture the measuring window of a refractometer in the shape of a prism, where all the surfaces met by light are plane surfaces. Furthermore, EP 0 359 167 known solution in which other geometric forms for the measuring window are used. German publication DE 10 2007 039 349 provides a manufacturing solution that involves measuring the amount of light reflected from the measuring surface so that the measuring window has a Fresnel lens.
It is also known in the field to have the critical angle of the total reflection expressed as a boundary of light and dark areas by directing the light reflected from the interface of the window and liquid by means of a lens system to a cell of a camera, for example. In known devices, the lens system is set to be at a distance of its focal length from the camera. Known refractometers direct light through prism-like surfaces to the interface of the liquid being measured and measuring window. Moreover, known refractometers direct light through prism surfaces serving as mirrors to the liquid interface.
As would be understood from known implementations, the operation of a refractometer is based on a most accurate angular measurement, because the critical angle of the total reflection is determined according to the refractive index of two substances. In known refractometers, problems have arisen, for example, due to the fact that the optics (e.g., lens arrangements) used in connection with the measuring window, and the light detector have been rigidly fixed to the frame structure of the device. Consequently, if the frame structure warps, for example, an error will result in the angular measurement. In the construction of a refractometer, the objective lens on the side of the detection of the critical angle should be as close as possible to the interface between the prism acting as the measuring window and the solution. A prism manufactured with plane surfaces makes the construction of the measuring device problematic. It has proven difficult to mechanically mount the lenses in a lens arrangement so that thermal movement or vibration do not change the place or position of the lenses when very small angular changes are measured. In particular, it is difficult to mount the objective lens in close proximity to the prism, which acts as the measuring window, in an adequately stable manner.